The __gthread_yield() function is only defined for gthreads targets, so
check _GLIBCXX_HAS_GTHREADS before using it.
Also reorder __thread_relax and __thread_yield so that the former can
use the latter instead of repeating the same preprocessor checks.
libstdc++-v3/ChangeLog:
* include/bits/atomic_wait.h (__thread_yield()): Check
_GLIBCXX_HAS_GTHREADS before using __gthread_yield.
(__thread_relax()): Use __thread_yield() instead of repeating
the preprocessor checks for __gthread_yield.
cc9a0a3d79